Underwater Light Field and its Comparison to Metal Halide Lighting

Over the past few years of
testing MH lamps used in the hobby and attempting to characterize
the light over the aquariums, one question that has often been at
the back of my mind has been – How does the light over our
aquariums compare to that on a natural reef? This article
presents some data on the underwater light field on a reef and
compares it to the artificial light field over our reef aquaria,
along with discussion of other features of natural lighting that
are often not simulated in our aquariums.

Surface Light Field

Before starting with the underwater light field, let us first
take a quick short look at light field at the surface of the
water along with the variations that are typically observed. The
typical Photosynthetic Photon Flux Density (PPFD) at sea level on
coral reefs on a clear sunny day have been measured at peak
levels of 1600-2300 μEinstein/m2/s (Maritorena et al.,
2002 . Typical variations result from several factors such as

  • the daily variation from sunrise to sunset and the change
    in elevation of the sun’s path across the sky
  • atmospheric factors such as scattering by air molecules and
    dust particles, absorption by water vapor and gases in the
  • level of cloud cover
  • seasonal variation (variation between summer and winter).
    The diurnal variation is higher for higher latitudes.

Figure 1 shows the spectral output of the incident sun light
over a Polynesian reef between the wavelengths of 400-700nm, the
visible range and the range used to compute the PAR or
Photosynthetic Photon Flux Density (data courtesy of S.
Maritorena). The incident PPFD around noon for sunny days was
typically around 2300 umoles/m2/s (500
watts/m2) in the summer and around 1600
umoles/m2/s (340 Watts/m2) in winter. The
day length varied from 11-13 hours, with daily incident energy in
the PAR range for clear skies increasing from 8.15 to 13.5
MJ/m2 between winter and summer.


Figure 1: Spectral Irradiance at the surface
of the water


Transmission Across the Air-Water Interface

The surface radiation at sea level enters the water at the
air-water interface. Some of it gets reflected back (depending on
the angle of incidence and the refractive index of water).
Fresnel’s equation determines the amount the light reflected
as a function of the angle of incidence and refractive index. The
angle of incidence varies throughout the day as the sun moves
across the sky. In addition, there are seasonal changes due to
the movement of the sun between the two tropics. The angle of the
sun impacts reef ecology – some reef organisms may be shaded
during some part of the day, and in bright light during other
parts of the day. This effect is more pronounced in shallower
waters. At greater depths the direction of light becomes less
sensitive to the sun’s position and moves toward the vertical
as depth increases and is more diffused with depth. So, the
direction of light reaching the deeper corals is relatively not
affected by the change in the sun’s position. However, the
intensity of light may change due to the relationship between the
angle of incidence and refraction.

Fresnel’s equation:


where r = reflectance


θa = angle of incidence in air

θw = angle of the transmitted beam, determined by
Snell’s law of refraction, and is given by θw =
θa /refractive index of sea water

From this equation it can be shown, that the reflectance
remains low about 2-3.3% for zenith angles ranging from 0-60
degrees, and rising rapidly after it (see Figure 2). The zenith
angle is the angle between the overhead point for an observer and
an object such as the sun. The solar zenith angle is zero if the
sun is directly overhead and is 90 degrees when the sun is on the
horizon. So for most of the day, there is very little light lost
due to reflectance at the air water interface if the sea is
relatively flat.

Figure 2: Reflectance of Light from Flat
Water Surface


In reality, the water surface is not flat, it is usually
roughened by the wind. However, this has very little effect on
the reflectance of sunlight at high solar elevations. At lower
elevations of the sun (sun on the horizon), the amount of photons
reflected at the surface increases rapidly at angles greater than
60 degrees (measured from the vertical), the rough surface caused
by the waves can lower such effect.


Surface waves also affect the light intensity. The waves can
act as a lens and focus the light, creating “glitter
lines.” These glitter lines appear as flashes of light of
high intensity and short duration, and have been measured as
having up to twice the intensity of light incident on the water
surface (Falkowski et al., 1990). These glitter lines occurs only
in direct sunlight, and do not occur when the light is diffused
(as when a cloud obscures the sun). Whether this flashing light
is advantageous to the corals or not is not known, but some
researchers have found that it enhances photosynthetic
performance in some unicellular algae (Falkowski et al. 1990). In
reef aquariums, these glitter lines can be created through the
use of point source lights (e.g., metal halide bulbs) and surface
agitation of the water. Florescent lighting is more diffuse and
does not create these effects. In an aquarium the light tends to
be directed downward through the use of the reflectors and hence
the angle of incidence is usually low. Thus we can expect the
loss due to reflection of the water surface to be reasonably low,
about 2-5%.

Underwater Light Field in Nature

Once the photons of light have made it all the way from the
sun and across the air-water surface on the coral reefs, lets see
what happens to them when they get into the water. The light
traveling from the top of the water surface toward the bottom is
called downwelling light. Some of the downwelling light
gets absorbed, and some is scattered by the dissolved and
particulate matter in the water and by the water molecules
themselves. Turbidity is a term used to describe the
amount of particulate matter. The higher the turbidity, the more
light that is absorbed and scattered. The absorption and
scattering result in reduction in the quantity of downwelling
light as the depth increases (Kirk, 1994). Once the photons
enters the water, most photos are eventually absorbed – either by
the light absorbing molecules of the water, the optically active
dissolved substances, the particulate matter, and eventually by
the photosynthetic process that occurs in the corals and the
suspended phytoplankton in the water column.

Further, the scatter also creates some upwelling
(backscattering of light). Some of this upwelling
light escapes out of the water back into the air and is the
reason for the color of the ocean. On coral reefs, the upwelling
irradiance is also increased by reflection from the
“white” calcium carbonate substrate found on the reef
floor. In fact, on coral reefs this upwelling irradiance may be a
significant portion of the total irradiance (Dustan 1982). This
upwelling light plays a critical role in allowing the growth of
corals on the under storey of the reefs. Thus, the addition of a
white calcium carbonate substrate in a reef aquarium also helps
in increasing the upwelling irradiance, while simultaneously
increasing the biodiversity. Rather than covering all the sand
with live rock, a good strategy would be to provide large open
sand areas to increase upwelling irradiance.

Natural waters have what are often referred to as inherent and
apparent optical properties. Inherent optical properties (IOP)
are a function of the water and optically active substances in it
and are not influenced by the geometric structure of the light
fields. IOP were usually determined in the laboratory but now
routinely measured in situ too and include the following (all
units are m-1):

  • absorption coefficient (a) – fraction of
    the incident flux absorbed divided by

    the thickness of an infinitesimally thin layer of medium
  • scattering coefficient (b) – fraction of
    the incident flux scattered divided by

    the thickness of an infinitesimally thin layer of medium
  • beam attenuation coefficient (c)
    fraction of incident flux which is absorbed

    and scattered divided by the thickness of an
    infinitesimally thin layer. The beam

    attenuation coefficient is the sum of the absorption and
    scattering coefficients:

c = a + b

Apparent optical properties (AOP) are derived from
measurements of natural light fields in a water body. They depend
on the geometry of the light fields and are related to absorption
and scattering. The most common of these properties is the
diffuse attenuation coefficient for downwelling irradiance
(Kd). Irradiance at a given depth (EZ) is a
function of the irradiance at the surface (E0), the diffuse
attenuation coefficient, and the depth interval (Z) according to
the following relationship, where e is the base of the natural



From this we can see that when light penetrates water its
intensity decreases exponentially with increasing depth.

The diffuse attenuation coefficient can thus be estimated by
taking measurements at different depths, and using the above
formula to compute Kd. The units for Kd are


Similarly to the absorption and scattering coefficients, the
diffuse attenuation coefficient is not independent of wavelength,
and is in fact different for different wavelengths. We can
compute this value Kdλ for different wavelengths if we
know the spectral curve at different depths. This spectral curve
is usually obtained using an underwater spectroradiometer.

Using the data from the Ocean close to a high island in French
Polynesia, lagoon of a high island (Moorea) and lagoon of an
atoll (Takapoto), the diffuse attenuation coefficient for these 3
waters is shown in the figure 3 (Maritorena, 1996). As seen from
this plot the 3 waters have slightly differing Kd
values at different wavelengths. Higher Kd values
imply that the light penetration will be less in that water.

Figure 3: Diffuse Attentuation Coefficient
for different waters


As light passes through the water column, it is attenuated
exponentially, and this attenuation is not uniform across all
wavelengths. So the water acts as a “filter,” reducing
the spectrum of light that penetrates it. As depth increases, the
waveband of light that penetrates narrows. The shorter
wavelengths (reds and yellows) are the first to be absorbed, and
the blue light penetrates the deepest.

Using these diffuse attenuation coefficients (obtained from
spectral irradiance measurements at different depths in the
water), and using the fact that the attenuation in water
decreases exponentially with respect to depth (given by the
equation discussed earlier), we can plot the spectral curve for
the underwater light field at various depths. Figure 4 shows the
spectral distribution of light under water using the
Kd values for clear ocean water during summer. The
PPFD values corresponding to these spectral plots as a function
of depth are given in the table below.

1m 5m 10m 15m 20m
PPFD 1640 958 618 436 316

Figure 4: Spectral Distribution of Light
Underwater at Different Depths


Aquarium Light Field

Previous articles on spectral analysis of MH lamps and light
distribution due to reflectors have provided data on the spectral
characteristics and PAR of the existing light field over the
aquarium. Taking 3 popular lamps that are representative of the 3
major classes of spectral distributions available to the
aquarist: the 400W Iwasaki 6500K and the Ushio 400W 10000K and
the Radium 400W 20000K, we can now make comparisons of these
metal halide lamps to the natural light field under water.
Keeping in mind that the change in intensity of a lamp only
scales the spectral distribution, we can create spectral plots of
PPFD equal to that of natural waters at different depths by
scaling the spectral plots of these lamps.

Figure 5: Comparison of 400W 6500K Iwasaki to
spectral distribution at varying depths


Figure 6: Comparison of 400W 10,000K Ushio to
spectral distribution at varying depths


Figure 7: Comparison of 400W 20,000K Radium
to spectral distribution at varying depths


Figures 5,6 and 7 shows how the spectrums of the 3 metal
halide lamps would compare to the natural waters for which the
data is provided. As seen from figure 5, the Iwasaki 6500K
provides a very good approximation of the spectrum underwater at
a depth of 1m. At other depths the spectral irradiance at
wavelengths less than 550 nm is less than natural water and
higher than natural water at wavelengths greater than 550nm. For
the 400W Ushio, it can be seen that at some wavelengths the
intensity far exceeds that of natural light, in some cases by
factors of 4-5 times what is observed in nature. The Radium shows
almost 8-12 times the radiation at 454 nm than what would be
observed in nature. Clearly other than the Iwaski which provides
a good spectral match to the underwater light field at 1 m, there
is a wide discrepancy in the spectral distribution of light over
aquariums as compared to the natural light field.

An interesting question to ask is “What is the
implication of this spectral discrepancy on the corals?”. I
do not have any definitive answers but can speculate. Corals are
highly adaptable in their ability to capture light and it is
quite likely that they regulate the light harvesting pigments to
adapt to the available light spectrum. This may possibly explain
why wild corals change colors in our aquariums, or even why the
coral color changes when grown under different light sources.

Effect of Water on the Light Field

Most of the SPS and other corals in our reefs are found in
waters less than 15-20 meters deep, but our reef aquariums are
usually only 24 to 30 inches deep. At this depth of water, the
amount of light lost to absorption by the water is quite small.
Using the diffuse coefficients for the ocean waters presented
above, the amount of light at 700nm absorbed by 2ft of water is
33% of light just below the surface. For light at 400nm this is
only 4%. So, we cannot rely on water to create large difference
in spectral distribution in the tank and have to rely on the
bulbs to provide a “correct” spectral distribution.
Further more in our aquariums, the primary cause of light drop
off is due to the “inverse square law”. This drop due
to the distance from the source of light is much higher than the
absorption in the water.

Figure 8 shows the light attenuation at depths equal to 2 ft
of aquarium water, assuming the Kd values for the
ocean water.

Figure 8: Attentuation of natural light at
aquarium depth of 2ft


Assuming the same Kd values, we can determine the
change in spectrum of a metal halide lamp from the surface of the
water to a depth of 2ft under water. Figure 9 shows the change in
spectrum of the Iwasaki 6500K lamp just due to the water. In our
reef aquariums this loss is quite small compared to the intensity
loss due to distance from the lamp and reflector. For the Iwasaki
lamp at 6″ from the water surface, the loss due to 2ft of
water is about 14%, where as the loss due to the change in
distance from the source (from 6″ to 30″) is about
70-90% based on the inverse square law of light and depending on
the reflector being used. In practice this loss will be lower due
to the additive effect due to multiple lights, but will still
dominate the light loss as compared to loss due to attenuation in
water assuming clear water similar to the ocean.

Figure 9: Attentuation of Ushio 4004W 6″
from the surface due to 2ft of water


Other Comparisons of Natural Light to Aquarium Light

The amount of light and spectrum absorbed in the aquarium is
often affected by the yellowing pigments or gilvin and the
suspended particulate matter, and in fact may be higher than what
we see in the ocean. The gilvin is typically made up of yellowing
substance derived from humic acids from the decomposition of
decaying plant material, run off from the terrestrial waters,
breakdown of phytoplankton, etc (Jerlov, 1976). The absorption
curve of gilvin follows an exponential curve with the absorption
being the lowest in the red end of the spectrum and rising with
decrease in wavelength towards the blue. Absorption is highest in
the UV-A wavelength. The impact of this can be quite significant
in an aquarium with yellowing water. Breakdown (and removal) of
some of the gilvin through the use of activated carbon and ozone
does, in fact, increase the irradiance transmitted through the
water, especially in the UV range (Bingman 1996).

The light reaching the corals is not constant. This is one
aspect of natural light that is not typically simulated in most
of our aquariums. We typically subject the corals to almost
constant light intensity with a fixed angle of incidence. The
movement of the sun can be simulated to some extent through the
use of light movers, which are available in the hydroponics
gardening industry. Light movers that move light in a circular
manner or linearly over rails may be used to provide the light
variability throughout the day. However, given the fact that most
home aquariums are less than 6 feet in length, the effect
produced by moving the lights will not be as significant as in

In addition to these variations, other natural variations are
introduced by meteorological and biological events. The clouds
over the reefs modulate the intensity of light. Storms may
directly reduce the amount of light reaching the reef and may
also stir up the substrate, increasing the turbidity and cutting
off light reaching the corals. Seasonal plankton blooms can also
increase turbidity of the water. Typically, most home aquariums
do not simulate these variations, although some aquarists do
introduce a “midday” cloud simulation by shutting off
some of the lights randomly during the day for a short period of
time. Other devices, such as light dimmers, are also becoming
available. These devices will dim the lights to about 25-40
percent and can also be programmed to create this simulated cloud
cover at various times during the day. Whether these variations
are beneficial to the corals has not yet been established.


The moon faithfully reflects the solar spectrum, so there is
no change in the spectrum of light emanating from the moon. The
intensity of solar radiation is about 500W/m2 while
the intensity of full moon is about 1mW/m2 (1 milli
Watt). Thus the intensity of a full moon is 0.5 Million times
less that that of noon sunlight. Since the spectrum is the same
as sunlight, the ocean waters will have the same diffuse
attenuation coefficient for moonlight as sunlight. So even though
the moon has the same spectrum of light as the sun the objects
illuminated by moonlight lack color, primarily due to the low
intensity of light and how the human eye responds at low

The reef aquarium industry and hobby has taken to using blue
LED’s to simulate moonlight. Cleary this is very different
from natural moonlight.


This article presented a comparison of natural underwater
light (based on specific data from natural waters) to the light
over our aquariums. While there are variations to be expected in
natural waters, the general shape of the spectral distribution
underwater will be quite similar. Clearly there is a wide
discrepancy between the spectral irradiance provided by MH and
the natural under water light field. We know from experience that
we can grow coral under all the 3 major classes of metal halide
lamps 6500K, 10000K and 20000K, so the corals must either adapt
to the spectrum or ignore the spectral quality. Unfortunately I
do not have any definitive answers to this, hopefully further
research will be able to provide more definitive answers.


  1. Bingman, C. 1996. Quantitative relationships between
    visible water color and ultraviolet transmission. Aquarium
  2. Falkowski, P. G., P. L. Jokiel and R. A. Kinzie. 1990.
    Irradiance and corals. In Ecoystems of the World, Volume 25
    Coral Reefs. Z. Dubinsky (Ed.):89-107.
  3. Jerlov, N. G. 1976. Marine Optics. Elsevier
    Scientific Pub., Amsterdam. Pp. 231.
  4. Kirk, J. T. 1994. Light And Photosynthesis In Aquatic
    Ecosystems, 2nd Edition
    . Cambridge University Press. Pp.
  5. Maritorena, S. 1996. Remote sensing of the water
    attenuation in coral reefs : A case study in French Polynesia.
    Int. J. Remote Sensing , 17(1) : 155-166.
  6. Maritorena, S., C. Payri, M. Babin, H. Claustre, L.
    Bonnafous, A. Morel and M. Rodire. 2002. Photoacclimatization
    in the zooxanthellae of Pocillopora verrucosa and comparison
    with a pelagic algal community. Oceanologica Acta. 25(3-4):
  Advanced Aquarist
Sanjay Joshi

 Sanjay Joshi

  (51 articles)

Sanjay Joshi in real life is a Professor of Industrial and Manufacturing Engineering at Penn State University. He has been a reef addict since 1992, and currently keeps several reef aquariums at home including a 500G SPS coral dominated reef. He also co-manages the 500G aquarium at Penn State. He has published several articles in magazines such as Marine Fish and Reef Annual, Aquarium Frontiers, Aquarium Fish, and Advanced Aquarist. In addition, he has been an invited speaker at several marine aquarium society meetings in the US and Europe. He received the MASNA award in 2006, for his contributions to the marine aquarium hobby.

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